AIDS prevention pill study halted; no benefit seen

The new study's result "must be seen as what it is - the closure of a single trial in a field that has generated exciting results in the recent past," said Mitchell Warren, head of the AIDS Vaccine Advocacy Coalition, a nonprofit group that works on HIV prevention research.

Two other large studies testing AIDS drugs for prevention are under way in Africa, in heterosexual women and in couples where one has the virus and the other does not. Results are expected within two years. The studies have mostly been in countries with high rates of new infections because that makes it easier to see whether a prevention measure is having an impact.

Truvada costs $5,000 to $14,000 a year in the United States, but as little as $140 a year in some poor countries where it's available in generic form.